This invention relates to fuel cell systems and, in particular, to integrated fuel cell and gas turbine systems having enhanced efficiency.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions.
A fuel cell may be combined with a heat engine such as a turbine generator to produce a high efficiency system, commonly called a hybrid system. In a conventional hybrid system, the fuel cell is typically situated in the position normally occupied by the combustor of the turbine generator so that air compressed by the turbine generator compressor section is heated and then sent to the fuel cell cathode section. In this arrangement, the fuel cell is operated at a high pressure, which substantially increases the cost of the power plant hardware and inhibits the use of internal reforming in the fuel cell. This further increases the plant cost and reduces efficiency, and subjects the fuel cell to potentially damaging pressure differentials in the event of plant upset. Furthermore, the fuel cell pressure is coupled with gas turbine pressure, limiting the application to system designs where the gas turbine pressure is nearly matched with the fuel cell pressure.
To overcome these disadvantages, another type of arrangement of a hybrid system has been developed, where a turbine generator is bottomed with a fuel cell so that the heated air discharged from the gas turbine is delivered to the cathode section of the fuel cell. U.S. Pat. No. 6,365,290, assigned to the same assignee hereof, discloses such a hybrid fuel cell/gas turbine system, in which waste heat from the fuel cell is used by a heat recovery unit to operate the heat engine cycle, and the system is arranged such that the compressed oxidant gas, heated in the heat recovery unit and by a heat exchanger, is expanded in the expansion cycle of the heat engine. It is then passed through an oxidizer which also receives the anode exhaust, passed through the heat exchanger and the resultant gas delivered to the cathode section of the fuel cell.
The aforesaid system of the '290 patent permits the fuel cell to be a high temperature fuel cell, while achieving a relatively high efficiency. However, the system also requires that the fuel cell size and the gas turbine size be matched in order to produce optimal efficiencies. This limits the usefulness of the system.
It is therefore an object of the present invention to provide an improved hybrid fuel cell/turbine system having greater flexibility in the balance of power between the fuel cell and the turbine.
It is a further object of the present invention to provide a hybrid fuel cell/turbine system which has enhanced efficiency and increased power output.